In a conventional semiconductor light emitting device that employs an InGaAlP based compound semiconductor, a first clad layer 2 made of an InGaAlP based semiconductor material, an active layer 3 made of a non-doped InGaAlP based semiconductor material having a composition wherein the band gap is smaller than that of the first clad layer, and a second clad layer 4 having the same composition as that of the first clad layer, respectively have been epitaxially grown on a semiconductor substrate 1 made of, for example, an n-type GaAs as shown in FIG. 3A so that a light emitting layer forming portion 9 of a double hetero-junction structure is formed. A window layer 5 made of an AlGaAs based semiconductor material having approximately the same refractive index as that of the clad layers, is further deposited on the front side of the light emitting layer forming portion 9, and furthermore a p-side electrode 7 made of an Au—Ge/Ni alloy is provided in a part of the surface of the window layer 5 via a contact layer 6 made of a p-type GaAs while an n-side electrode 8 made of an Au—Ge/Ni alloy is provided on a rear side of the semiconductor substrate 1. And a wafer in which the above processes are performed is divided into chips.
Carriers are confined in the active layer by adopting a double hetero-structure in the conventional structure, and a difference of refractive indexes is formed in the direction perpendicular to a surface of the substrate so that light is confined in the active layer having a refractive index higher than that of the clad layers. Though no problem arises in a semiconductor laser that emits light from an edge surface wherein carriers and light are confined in the active layer, in the case of an LED in which light is emitted from a top surface of an LED as shown in FIG. 3A, light cannot be effectively emitted from the top surface, if light is confined too strongly in the active layer.
On the other hand, in order to solve such a problem, an LED chip has been proposed, which has a structure wherein exuding of light from the active layer is increased so as to increase the amount of light that proceeds to the top surface and increases the efficiency of the emission of light from the top surface (hereinafter referred to as “external quantum efficiency”) by making the active layer a thin film, as shown in FIG. 4A. In this case, however, the light released to the substrate from the active layer is simultaneously increased when the exuding of light is increased. Light emitted from the active layer to the substrate is absorbed by the GaAs substrate and the external quantum efficiency is not substantially increased in the case wherein, for example, a GaAs substrate having the band gap smaller than that of the active layer is used.
The usage of a GaP substrate which does not absorb light while making the active layer thinner as described above is described in “1.4×efficiency improvement in transparent-substrate (AlxGa1-x)0.5In0.5P light-emitting diodes with thin (≦2000 Å) active regions” by N. F. Gardner et al, Applied Physics Letters Volume 74 No. 15 pp 2230–2232, Apr. 12th 1999. In this case, however, a problem arises wherein the manufacturer requires an adhesion process which is significantly complicated and is very costly.
Furthermore, the current density increases in the active layer and temperature increases in the active layer to affect the reliability of the device in the case wherein the active layer is made into a thin film.